Hardcover? Or paperback?
Illustrations? Or just text?
First editions? Or you don’t care?
Signed by the author? Or not?
My attitude here can be summed up in a single word: Whatever. If it has a cover that’s not too ugly and a full complement of pages, and it’s affordable, I’m happy. And if it’s not on the verge of falling apart, though I have gotten good at patching up tattered spines. That being said, while hardbacks can be nice to look at I do prefer paperbacks (the small ones, not trade) simply for portability.
I love pictures in non-fiction books; they’re one of the first things I look for and I often refer to them while reading. In fiction, though, the only kind of illustration I care about is maps. I just finished Robert Harris’s Pompeii and the absence of a map drove me nuts - I ended up drawing my own from ones on the net. It’s always nice to be able to get my head around the geography of the place I’m vicariously inhabiting.
As for first editions and signatures - I’m not an autograph hunter, and these are things it wouldn’t even occur to me to look for.